Method of forming vias in silicon carbide and resulting devices and circuits

ABSTRACT

A method of fabricating an integrated circuit on a silicon carbide substrate is disclosed that eliminates wire bonding that can otherwise cause undesired inductance. The method includes fabricating a semiconductor device on a first surface of a silicon carbide substrate and with at least one metal contact for the device on the first surface of the substrate. The opposite, second surface of the substrate is then ground and polished until it is substantially transparent. The method then includes masking the polished second surface of the silicon carbide substrate to define a predetermined location for a via that is opposite the device metal contact on the first surface; etching the desired via through the desired masked location until the etch reaches the metal contact on the first surface; and metallizing the via to provide an electrical contact from the second surface of the substrate to the metal contact and to the device on the first surface of the substrate.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of copending U.S. application Ser.No. 09/546,821, filed Apr. 11, 2000, now U.S. Pat. No. 6,475,889.

The present invention relates to integrated circuits formed insemiconductor materials and in particular relates to methods for formingvia openings in semiconductor substrates and the resulting structures.More particularly, the invention relates to the use of such vias to formmonolithic microwave integrated circuits (MMICs) in silicon carbide(SiC).

BACKGROUND OF THE INVENTION

The present invention relates to the manufacture of via openings(“vias”) in integrated circuits (ICs), and in particular relates to amethod of forming such vias in silicon carbide in order to takeadvantage of silicon carbide's electronic, thermal, and mechanicalproperties in the manufacture and use of monolithic microwave integratedcircuits.

MMICS

In its most basic sense, a monolithic microwave integrated circuit is anintegrated circuit; i.e., a circuit formed up a plurality of devices; inwhich all of the circuit components are manufactured on top of a singlesemiconductor substrate, and which is designed to operate at microwavefrequencies. As is generally the case with integrated circuits, theadvantage of placing the device and circuit components on a singlesubstrate is one of saving space. Smaller circuit size offers numerousadvantages for electronic circuits and the end-use devices thatincorporate such circuits. In general, the end use devices can besmaller while offering a given set of functions, or more circuits andfunctions can be added to devices of particular sizes, or bothadvantages can be combined as desired. From an electronic standpoint,integrated circuits help reduce or eliminate problems such as parasiticcapacitance loss that can arise when discrete devices are wire-bonded toone another to form circuits. These advantages can help integratedcircuits operate at improved bandwidths as compared to circuits that are“wired” together from discrete components.

Wireless communications systems represent one area of recent and rapidgrowth in integrated circuits and related commercial technology. Suchsystems are exemplified, although not limited to, cellular radiocommunication systems. One estimate predicts that the number of wirelesssubscribers for such phones will continue to grow worldwide and willexceed 450 million users in the immediate future. The growth of suchtechnologies will require that devices are smaller, more powerful andeasier to manufacture. These desired advantages apply to base, relay andswitching stations as well as to end user devices such as the cellularphones themselves.

As recognized by those of ordinary skill in this art, many wirelessdevices, and in particular cellular phone systems, operate in themicrowave frequencies of the electromagnetic spectrum. Although the term“microwave” is somewhat arbitrary, and the boundaries between variousclassifications or frequencies are likewise arbitrary, an exemplarychoice for the microwave frequencies would include wavelengths ofbetween about 3,000 and 300,000 microns (μ), which corresponds tofrequencies of between about 1 and 100 gigahertz (GHz).

As further known by those of ordinary skill in this art, theseparticular frequencies are most conveniently produced or supported bycertain semiconductor materials. For example, although discrete (i.e.,individual) silicon (Si) based devices can operate at microwavefrequencies, silicon-based integrated circuits suffer from lowerelectron mobility and are generally disfavored for frequencies aboveabout 3-4 Ghz. Silicon's inherent conductivity also limits the gain thatcan be delivered at high frequencies.

Accordingly, devices that operate successfully on a commercial basis inthe microwave frequencies are preferably formed of other materials, ofwhich gallium arsenide (GaAs) is presently a material of choice. Galliumarsenide offers certain advantages for microwave circuits and monolithicmicrowave integrated circuits, including a higher electron mobility thansilicon and a greater insulating quality.

Because of the frequency requirements for microwave devices andmicrowave communications, silicon carbide is a favorable candidatematerial for such devices and circuits. Silicon carbide offers a numberof advantages for all types of electronic devices, and offers particularadvantages for microwave frequency devices and monolithic microwaveintegrated circuits. Silicon carbide has an extremely wide band gap(e.g., 2.996 electron volts (eV) for alpha SiC at 300K as compared to1.12 eV for Si and 1.42 for GaAs), has a high electron mobility, isphysically very hard, and has outstanding thermal stability,particularly as compared to other semiconductor materials. For example,silicon has a melting point of 1415° C. (GaAs is 1238° C.), whilesilicon carbide typically will not begin to disassociate in significantamounts until temperatures reach at least about 2000° C. As anotherfactor, silicon carbide can be fashioned either as a semiconductingmaterial or a semi-insulating material. Because insulating orsemi-insulating substrates are often required for MMICs, this is aparticularly advantageous aspect of silicon carbide.

MMICs are fabricated with backside metallic ground planes, to whichcontacts must be made from various points in the MMIC, for example attransmission line terminations. Traditionally, this has beenaccomplished by wire bonds. Although wire bonding techniques can be usedfor other devices that operate at other frequencies, they aredisadvantageous at microwave frequencies in silicon carbide devices. Inparticular, wires tend to cause undesired inductance at the microwavefrequencies at which silicon carbide devices are capable of operating.For frequencies above 10 GHz, wire bonding simply must be avoidedaltogether. Accordingly, such wire bonding is desirably—and sometimesnecessarily—avoided in silicon carbide-based MMICs.

The use of conductive vias (i.e., via openings filled or coated withmetal) to replace wire bonds is a potential solution to this problem. Todate, however, opening vias in silicon carbide has been rather difficultbecause of its extremely robust physical characteristics, which, asnoted above, are generally advantageous for most other purposes.

Etching and Etchants

Etching is a process that removes material (e.g., a thin film on asubstrate or the substrate itself) by chemical or physical reaction orboth. There are two main categories of etching: wet and dry. In wetetching, chemical solutions are used to etch, dry etching uses a plasma.Silicon carbide does not lend itself rapidly to wet etching because ofSiC's stability and high bond strength. Consequently, dry etching ismost often used to etch silicon carbide.

In dry etching, a plasma discharge is created by transferring energy(typically electromagnetic radiation in the RF or microwave frequencies)into a low-pressure gas. The gas is selected so that its plasma-stateetches the substrate material. Various fluorine-containing compounds(e.g., CF₄, SF₆, C₄F₈) are typically used to etch silicon carbide anddifferent plasma reactor systems may also use gas additives such asoxygen (O₂), hydrogen (H₂), or argon (Ar). The plasma contains gasmolecules and their dissociated fragments: electrons, ions, and neutralradicals. The neutral radicals play a part in etching by chemicallyreacting with the material to be removed while the positive ionstraveling towards a negatively charged substrate assist the etching byphysical bombardment.

Reactive ion etching (RIE) systems typically use one RF generator. TheRF power is fed into one electrode (the “chuck,” on which the wafers areplaced), and a discharge results between this electrode and the groundedelectrode. In such systems, the capacitive nature of RF energy couplinglimits the density of the plasma, which in turn leads to lower etchrates of silicon carbide. In RIE systems, plasma density and ion energyare coupled and cannot be independently controlled. When RF input powerincreases, plasma density and ion energy both increase. As a result, RIEsystems cannot produce the type of high density and low energy plasmafavorable for etching vias in silicon carbide.

In inductively coupled plasma (ICP) systems, two RF generators are used.One feeds RF power to a coil wrapped around the non-conductive dischargechamber. The second feeds power to the electrode (chuck) on which thewafers are placed. In such systems, the inductive nature of the RFenergy coupling increases the efficiency of energy coupling and hencethe density of the plasma. Additionally, the plasma density can beindependently controlled by the coil RF power, while the ion energy canbe independently controlled by the chuck RF power. Thus, ICP systems canproduce the high density and low energy plasmas that are favorable foretching vias in silicon carbide.

Etches are performed on selected areas of the wafer by masking areas ofthe wafer that do not need to be etched. The ratio of the etch rate ofthe substrate (the material to be etched) to the etch rate of the maskmaterial is referred to as the “selectivity” of the etch. For deepetches and faithful pattern transfer, high selectivity etches aredesired.

Etches generally proceed in both the vertical and horizontal directions.The vertical direction can be measured as etch depth in the unmaskedareas, while the horizontal direction can be measured as undercut underthe mask areas. The degree of anisotropy is expressed by how much theratio of the horizontal etch rate to the vertical etch rate deviatesfrom unity. When the etch rate in the vertical direction is much greaterthan the rate in the horizontal direction, the etch is calledanisotropic. The reverse characteristic is referred to as beingisotropic. Because of silicon carbide's high bond strength, it does notetch without ion bombardment in the horizontal direction. As a result,dry etches of silicon carbide are generally anisotropic.

In contrast, etches of silicon (Si) in ICP systems are generallyisotropic. This results from silicon's low bond strength, because ofwhich it readily etches in the horizontal direction. Silicon etches canbe made anisotropic by using the Bosch process that alternates adeposition step for sidewall protection and an etch step.

The use of ICP (inductively coupled plasma) and ECR (electron cyclotronresonance) sources for SiC etching have resulted in higher etch rates ascompared to RIE (reactive ion etch). Both ICP and ECR systems use loweroperating pressure (e.g., 1 to 20 milliTorr), higher plasma density(10¹¹ to 10¹² cm⁻³) and lower ion energies compared to RIE systems. Thecombination of these parameters result in high etch rate of SiC andminimal erosion of the etch mask. RIE systems use higher pressure (10 to300 milliTorr) lower plasma density (10¹⁰ cm⁻³) and higher ion energiesto break SiC bonds and etch; however, the detrimental effects of highion energies and low plasma density include mask erosion and lower etchrate.

As reported in the scientific literature by McDaniel et al., Comparisonof Dry Etch Chemistriesfor SiC, J. Vac. Sci. Technol. A., 15(3), 885(1997), scientists have been successful in etching SiC using an electroncyclotron resonance (ECR) plasma. Scientific studies have determinedthat higher ion density ECR discharges of CF₄/0₂ or SF₆/0₂ results in amuch higher etch rate than RIE. In contrast with RIE, there have been noobserved benefits to adding oxygen to either NF₃ or SF₆ during ECRetching.

Previous attempts at using plasma chemistries for high-density plasmaetching of SiC include the use of chlorine (Cl₂), bromine (Br₂), oriodine (I₂)-based gases. However, the use of fluorine-based gas hasproduced much higher etch rates. For example, Hong et al., PlasmaChemistries for High Density Plasma Etching of SiC, J. ElectronicMaterials, Vol. 28, No. 3, 196 (1999), discusses dry etching of 6H-SiCusing a variety of plasma chemistries which include sulfur hexafluoride(SF₆), chlorine (Cl₂), iodine chloride (ICl), and iodine bromide (IBr)in high ion density plasma tools (i.e., ECR and ICP). These efforts haveachieved etch rates of around 0.45 μm/minute (4500 Å/minute) with SF₆plasmas. Alternatively, Cl₂, ICl, and IBr-based chemistries in ECR andICP sources resulted in lower rates of 0.08 μm/minute (800 Å/minute). Itwas found that fluorine-based plasma chemistries produced the mostrapid, and hence most desirable, etch rates for SiC under high-densityplasma conditions. Unfortunately, the fluorine-based chemistriesdisplayed a poor selectivity for SiC with respect to photoresist masks.

Wang et al. reported in Inductively Coupled Plasma Etching of Bulk6H-SiC and Thin-film SiCN in NF ₃ Chemistries, J. Vac. Sci. Technol. A,16(4) (1998) the etching characteristics of 6H p+ and n+ SiC andthin-film SiC_(0.5)N_(0.5) in inductively coupled plasma NF₃/O₂ andNF₃/Ar discharges wherein etch rates of 0.35 μ/minute (3,500 Å/minute)were achieved.

In further scientific literature, Cao et al., Etching of SiC UsingInductively Coupled Plasma, J. Electrochem. Soc., Vol. 145, No. 10(1998) discusses plasma etching in an ECR plasma using CF₄ and O₂ gas atflow rates of 20 standard cubic centimeters per minute (sccm) and 9sccm, respectively, attained an etch rate in SiC of about 0.05 μm/minute(500 Å/minute). The process resulted in a 14 μm deep trench having asmooth bottom surface. Further, the low chamber pressure (i. e., 7mTorr) minimized micromasking effects during the deep etch trenching.During the Cao et al. investigation, substrate bias was maintained at 10V and the coil power was maintained at 700 W.

In view of the technologies discussed above, a primary objective of SiCvia etching is finding a process in which SiC is etched at a reasonablerate while erosion of the etch mask is kept to a minimum. The factorsaffecting this objective are the choice of mask material, plasmachemistry, plasma density, and ion energy. A secondary objective whenetching vias in SiC is obtaining smooth etch surfaces.

Therefore there is a need for a process in which SiC may be etched at areasonably rapid rate while erosion of the etch mask is minimized.

There is also a need for a method for etching a via in SiC of sufficientdepth and at a reasonable rate which results in a smooth surface at thebottom of the via trench.

A further need exists for a technique that successfully incorporates theuse of appropriate vias in semi-conducting silicon carbide substrates tofacilitate the manufacture of silicon carbide based MMICS and the enduse devices that can be formed with the silicon carbide-based MMICS.

OBJECT AND SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provide a methodof etching vias in and entirely through silicon carbide substrates, in amanner which favorably differentiates between the silicon carbide beetched and the masking material.

The invention meets this object with a method of etching a via on asilicon carbide substrate that has first and second surfaces on oppositesides of the substrate. The method comprises placing a conductive etchstop material at a predetermined position on a first surface of asilicon carbide substrate, masking the second surface of the siliconcarbide substrate to define a predetermined location for a via that isopposite from the predetermined position for the conductive etch stopmaterial, etching a via in the substrate from the masked second surfaceuntil the etched via reaches entirely through the substrate to theconductive etch stop material, and connecting the conductive etch stopmaterial on the first surface of the substrate to the second surface ofthe substrate.

In another aspect, the invention comprises the method of fabricatingintegrated circuits on silicon carbide substrates while reducing theneed for wire bonding that can otherwise cause undesired inductance athigh frequencies.

In another aspect, the invention comprises a circuit precursorcomprising a silicon carbide substrate having respective first andsecond surfaces, a via extending entirely through the silicon carbidesubstrate, and a conductive contact through the via connecting the frontand back surfaces of the silicon carbide substrate.

In yet another aspect, the invention is a Monolithic MicrowaveIntegrated Circuit (MMIC) comprising a semi-insulating silicon carbidesubstrate having respective opposite first and second surfaces, amicrowave circuit formed on the first surface of the substrate, thecircuit including a plurality of conductive contacts on the firstsurface, a plurality of vias extending entirely through the substratewith each of the vias terminating at one of the conductive contacts, anda conductor in each via for forming a complete electrical pathwaybetween the first and second surfaces of the silicon carbide substrate.

These and other objects and advantages of the invention, and the mannerin which the same are accomplished, will be more fully understood whentaken in conjunction with the detailed description and drawings inwhich:

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 through 11 are cross-sectional diagrams illustrating the methodof forming a via through a silicon carbide substrate to a device inaccordance with the present invention; and

FIG. 12 is a scanning electron micrograph (SEM) of a via formed in asilicon carbide substrate according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a first aspect, the invention is a method of forming vias in aparticular material—silicon carbide—that enables integrated circuits,and particularly monolithic microwave integrated circuits, to be formedin silicon carbide substrates in a manner that reduces the inductanceproblems that are characteristic of such MMICS when wire bonding is usedto form electrical contacts for high frequency devices and circuits.

FIGS. 1 through 11 illustrate in sequential fashion the basic steps ofthe method aspects of the present invention. These will be describedsomewhat generally, following which particular experimental details willbe set forth. Because much of the background of MMICs and their functionis well understood in this art, these will not be described inparticular detail other than as necessary to highlight the invention. Inthe same manner, certain process steps are generally well understood sothat where appropriate, these will be simply named rather than describedin detail. The novel and non-obvious features of the invention, however,are set forth herein in sufficient detail to provide those referring tothe specification with the ability to carry out the inventionsuccessfully and without undue experimentation.

FIG. 1 is a cross sectional view of a silicon carbide substrate 20 thatincludes a device portion indicated by the brackets 21. As stated above,the purpose of the present invention is to form a via in the SiCsubstrate 20, and to use the via to provide an electrical path throughthe substrate 20 and to the device 21 For descriptive purposes, FIGS. 1through 11 illustrate a single via to one contact of a single device. Itwill nevertheless be understood that the method of the invention, andthe resulting structure, are more typically applied to forming numerousvias to numerous devices that form a circuit. Certain of the methodsteps of the invention are, however, most clearly set forth bysimplifying the illustrations.

Accordingly, FIG. 1 is meant to illustrate in broad fashion a devicesuch as a metal-semiconductor field-effect transistor (MESFET) with anappropriate source portion 22, gate portion 23, and drain portion 24. Inpreferred embodiments, particularly for microwave frequency devices, thesource 22, gate 23, and drain 24 are all formed in a wide band gapmaterial such as silicon carbide, or certain of the III-V nitrides suchas gallium nitride (GaN) and related binary, ternary, and tertiary III-Vcompounds such as AlGaN and InAlGaN.

The device is formed on a silicon carbide substrate 20 that hasrespective first and second surfaces 26 and 27.

FIG. 2 illustrates the same device as FIG. 1, but with a conductivecontact 25 in place on the first surface 26. Those familiar with devicessuch as MESFETS will immediately recognize that an exemplary device mayalso include a contact to the gate 23 and another to the drain 24. Asjust noted, however, such contacts are not shown in FIGS. 1-11 for thepurpose of simplifying the presentation of the relevant information.Accordingly, FIG. 2 simply shows the conductive contact 25 to the sourceregion 22 of the illustrated device. It will be understood that when thedevice is formed entirely in a single portion of silicon carbide, theentire portion can be considered the substrate 20.

Alternatively, and without departing in any manner from the invention,the substrate can also include one or more epitaxial layers(“epilayers”) in which the device portion 21 is formed. In suchembodiments, the first surface 26 would refer to the top surface (in theorientation of FIGS. 1-4) of the epitaxial layer. Those familiar withthe growth of semiconductor materials, and particularly the growth ofsilicon carbide, will recognize that the use of a substrate and anepitaxial layer (or layers) even though made of the same material,provides a method of (in most circumstances) gaining an improved crystallattice in the epitaxial layers (epilayers) as compared to thesubstrate. Because the use of epilayers, their method of manufacture,and the resulting structures are well understood in the art, and becausethe differences are minimal or non-existent with respect to the presentinvention, the word “substrate” will be used herein to refer to thesestructures, with the understanding that the structures can also includeepitaxial layers.

Accordingly, FIG. 2 shows that the conductive etch stop material, whichin the preferred embodiment is indium-tin-oxide (“ITO”) contact 25 isplaced at a predetermined position on the first surface 26 of thesilicon carbide substrate 20.

FIG. 3 illustrates that in order to provide a high quality contact fordevice, the indium-tin-oxide contact 25 is typically further coated witha noble metal 30 which in preferred embodiments is typically gold.

At this point, one of the particular advantages of the invention can behighlighted: the use of conductive ITO as the etch stop eliminates theneed to add and remove another etch stop material before and after theetch step respectively. Instead, the ITO is simply incorporated into thedevice or circuit before the via is etched. Because the ITO etch stopdoes such double duty, fewer materials need be introduced into theprocess environment, and fewer process steps are required. As knownthose familiar with semiconductor manufacturing techniques, processesusing fewer steps and fewer materials, yet producing the desiredstructures, are generally advantageous. Furthermore, eliminating aforeign etch stop material that would otherwise have to be both addedand then removed, is particularly advantageous.

FIG. 4 illustrates that in preferred embodiments, the device, againillustrated in simplified fashion by the source 22, the gate 23, thedrain 24, and the source contact 25, is covered with a protectivepolymer layer 31 which in preferred embodiments is a polyimide. Thepolyimide layer 31 protects the device underneath, and provides aleveling effect for the precursor for appropriate handling in thefollowing manufacturing steps.

FIG. 5 illustrates that in a next step, the polymer-coated first surface26 of the substrate 20 is mounted on a platen 32. The platen ispreferably forned of silicon carbide, in this case for its mechanicaland thermal properties rather than its electronic advantages. Typically,a mounting adhesive 33 is used to fix the polyimide coated surface 31 tothe platen 32. The mounting adhesive can be any appropriate materialthat will keep the polyimide-coated device and substrate fixed to thesilicon carbide platen 32 during the subsequent processing steps whileboth withstanding those steps and avoiding any interference with them.Such adhesives are generally well known in the art and will not bedescribed in detail herein.

FIG. 6 illustrates that in the next step of the preferred method, thesemiconductor substrate 20 is ground and polished until it issubstantially transparent. The grinding and polishing are carried outfor at least three reasons. First, because etching through siliconcarbide is difficult under any circumstances, minimizing the thicknessof the silicon carbide substrate 20 helps facilitate the overall etchingprocess. Second, by grinding and polishing the substrate 20 until it issubstantially transparent, an appropriate optical path can be definedfrom the second surface 27 of the substrate 20 to the metal contact 25so that appropriate positions for the vias can be aligned and etched tothe contact 25 in the desired manner, as described herein with respectto the remaining drawings. Third, the resulting thinner substrate (i.e.,less mass) offers thermal advantages for the resulting device or MMIC.

According to the present invention, when etching a via the front sideetch stop pads should be conductive so that the multiple layers formingthe integrated circuit will be connected, thereby allowing the circuitto perform its desired function. Further, the etch mask on the backsideof the sample is preferably transparent to permit optical alignment(including visual alignment) of the sample with the front side etchstop.

FIG. 7 illustrates that in the next steps of the preferred method of theinvention, the second surface 27 of the substrate 20 is coated with alayer 34 of indium-tin-oxide (ITO). The ITO is selected and incorporatedfor at least two reasons. First, the ITO layer 34 can be formed to betransparent, so that the method of the invention can incorporate typicalmicrolithography and masking techniques used in semiconductor design andmanufacture. Second, and as discussed in the Experimental section tofollow herein, the ITO provides a good masking material for SiC becausethe desired etchants discriminate as between SiC and ITO in a mannerthat is both desired and necessary during the etching process.

In another embodiment, the layer 34 on the substrate's second surface 27can comprise magnesium oxide (MgO), which offers the sameadvantages—selectivity and transparency—as ITO. As known to thosefamiliar with MgO, it can be produced in a very dense form with a veryhigh melting point (2800° C.).

The ITO layer is then coated with an appropriate photoresist layer 35.Photoresist compounds are generally well known in the art and will notbe otherwise discussed in detail herein, other than to note that anappropriate photoresist material should be compatible with deposition onthe ITO layer 34, should provide an appropriate level of definition whenexposed and developed, and should not otherwise interfere with theprocess steps that take place while the photoresist is in place.

FIG. 8 illustrates the precursor structure after the photoresist 35 hasbeen masked, exposed, and developed, steps which can otherwise becarried out in conventional fashion provided they are consistent withthe remainder of the process and materials. Opening the photoresistforms a defined opening 36 in the photoresist layer 35 through which theITO layer 34 can be appropriately opened and then, as illustrated inFIG. 9, the appropriate via 37 can be formed. In preferred embodimentsof the invention, the ITO layer is etched with a reactive ion etch usingwith boron trichloride (BCl₃) chemistry.

As FIG. 9 illustrates, the via is formed by an etching step, theparticulars of which will be described shortly hereinafter, but which ispreferably carried out in an inductively coupled plasma, and mostpreferably one formed from sulfur hexafluoride (SF₆).

In a particularly advantageous step, the method of the inventionincorporates the original conductive contact 25 as the etch stop. Inthis manner, the method of the invention avoids using additionalsteps—and (often just as importantly) additional materials—to add andthen remove a separate etch stop. Again, it is to be understood thatalthough the Figures illustrate only one via, such is for the purpose ofclarifying the illustrations, and the invention is advantageously usedfor opening multiple vias.

FIG. 10 illustrates that in preferred embodiments, the via is firstsputter-coated with three layers of metal: titanium, platinum, and gold(“Ti/Pt/Au”), in that order, along the floor and walls of the etchedtrench. This coating is designated as 40 in FIG. 10. The coating 40 isthen electroplated with a noble metal 41, preferably gold, to form thecomplete contact from the second surface 27 of the substrate 20 throughto the first surface 26, and more particularly to the contact 25 whichis part of the device portion 21. In preferred embodiments, theindium-tin-oxide layer 34 and the photoresist layer 35 are both removedprior to the step of sputter coating with the Ti/Pt/Au coating 40 andthe electroplating with the gold 41. The device precursor is thenremoved from the platen 32 and the protective polyimide layer 31 isstripped to produce the resulting device illustrated in FIG. 11.

FIG. 12 is an SEM micrograph of a 100 micron diameter via hole etched ina 4 mil (1000 mil=1 inch) silicon carbide wafer according to the presentconvention. Although FIGS. 1-11 are drawings and FIG. 12 is aphotograph, by way of comparison, the top surface illustrated in FIG. 12corresponds to the second surface 27 in the drawings. As understood bythose of skill in this art, the ability to put vias of this diameter insilicon carbide substrates of this thickness, makes broadband, highfrequency MMICS possible in desirable silicon carbide substrates.

The invention is a method of etching vias, typically (although notnecessarily limited to) about 25 to 200 microns in diameter through asilicon carbide substrate, 100 to 200 microns thick. The inventive etchprocess yields an etch rate of between about 0.5 and 0.8 microns perminute (μ/min.), a selectivity to the etch mask of 150, and anisotropyof 90 to 99%.

The central issue of etching vias in silicon carbide is finding an etchprocess which etches silicon carbide—a material of high stability andhigh bond strength—a reasonable rate (e.g., 0.5μ/min) while minimizingthe erosion of the etch mask.

The invention satisfies these diametrically opposing requirements by thechoice of mask material, plasma parameters, and chemistry.

In the invention, indium-tin-oxide (“ITO”) is the preferred etch maskfor vias in silicon carbide for several reasons. First, ITO is stableand does not etch in the fluorine chemistry that is most efficient andpreferred for etching silicon carbide. Second, unlike other hard metalmasks, ITO does not sputter at the ion energies that are sufficient tobreak silicon carbide bonds, and thus can etch silicon carbide. Third,ITO is also transparent, which allows the etch mask to be alignedthrough the wafer to the edge pads. Fourth, ITO is also used for theetch stop, because it is conductive and a can serve as the material onwhich the etch stops.

As noted above, one of the best etch masking materials for vias etchesin silicon carbide is Indium-Tin-Oxide (ITO). The ITO etch mask ispatterned as follows. The wafer is first blanket coated with ITO, thenwith photoresist. The photoresist is exposed through a mask with UVlight and the exposed areas harden, thus transferring the mask patternonto the photoresist. The photoresist acts as a mask in the subsequentetch of the ITO in the chlorine chemistry, thus transferring the patternof the photomask onto the ITO. The ITO then acts a mask in thesubsequent etch of the silicon carbide vias in fluorine chemistry.

An inductively coupled plasma (ICP) is used in the invention to generatea high density SF₆ plasma to etch vias in silicon carbide for severalreasons. First achieving a high etch rate in the silicon carbide whileminimizing the erosion of the etch mask requires a high density and lowenergy plasma. The use of ICP is critical for this purpose because itallows a high density plasma to be generated, and it permits theindependent control of plasma density by adjusting the coil power andion energy by adjusting the chuck power. A high coil power (600-1500 Wwith about 800 W preferred) is selected to maximize plasma density.

An important point of the invention is the use of a chuck power in theICP system that maximizes the etch rate of the silicon carbide whilekeeping the erosion of the ITO or MgO etch mask minimal. As the chuckpower is increased in an ICP system, the etch rate of the siliconcarbide increases; this increase, however, is much more drastic at lowchuck powers than at high chuck powers. As the chuck power is increasedthe erosion rate of the ITO or MgO etch mask is minimal and does notincrease initially; at higher chuck powers, however, it increasesrapidly. Thus, the invention incorporates the recognition that a chuckpower can be selected that on one hand maximizes SiC etch rate and onthe other hand maximizes the difference in the etch rates of SiC and ITOetch mask. In preferred embodiments, this chuck power level isdetermined to be between about 1 to 2 watts power square centimeter(Wcm⁻²) presently being preferred.

Sulfur Hexafluoride (SF₆) chemistry is used by the invention to etchvias in silicon carbide, because it is deemed to be the most efficientof the fluoride chemistries for such purpose. The invention does not useany gas additive, as it tends to slow down the etch rate of the siliconcarbide and speeds up the mask erosion by sputtering. The invention usesSF₆ at a pressure of 1 to 5 milliTorr, with about 3 mT being preferred.Similarly, the gas is supplied at a rate of between about 5 and 100sccm, with about 10 sccm being preferred.

In further investigations employing the use of SF₆, it was determinedthat SF₆ yielded a higher SiC to ITO mask selectivity (approximately150:1) than NF₃ or CF₄ (approximately 70:1). As discussed above,conventional methods of etching SiC included the use of NF₃ or SF₆diluted with Ar and CF₄/O₂. Upon investigation, however, the addition ofAr or O₂ to SF₆ or NF₃ reduced the etch rate in SiC and increased maskerosion due to the lower percentage of fluorine and greater ionbombardment. Thus, the use of SF₆ without additional gases is preferablein achieving the increased etch rate and high selectivity with respectto an ITO mask of the present invention.

The etch rate of a via can be increased by raising the temperature ofthe substrate or thin film applied thereon. Elevations in temperaturemay be achieved by halting the flow of helium to the backside of thesample, which serves to cool the sample. Otherwise, the backsidepressure is maintained at between about 1 and 10 torr. The chemicalreactions affecting the etch rate (e.g., breaking of molecular bonds)can also be increased by increasing the gas flow and chamber pressure.

An increase in the chemical reactions affecting the via etch results inan increased lateral etch and, thus, sidewall slope of the via. Theincrease in the chemical reactions also leads to an increase in the etchrate and erosion of the ITO mask. Further, spiking and surfaceimperfections may result from the enhanced chemical reactions.

As exemplified by the referenced cited in the Background, the equipmentand processes used to generate inductively coupled plasmas are generallywell-known and well-understood in this art. Accordingly, the techniquesdescribed herein can be carried out by those of ordinary skill in thisart, and without undue experimentation.

Experimental

In preferred embodiments, the present invention also comprises a methodof dry etching a via in SiC using sulfur hexafluoride chemistry in aninductively coupled plasma (ICP). In a particular embodiment of theinvention, the dry etching was conducted in a Model 790 ICP systemmanufactured by Plasma-Therm Incorporated.

In this system, the wafer is placed on a He-cooled chuck in the processchamber, the wafer is clamped and subsequently the process chamber isevacuated to 10⁻⁵ Torr with a turbo and mechanical pump. Five to twentycubic centimeter per minute electronic grade sulfur hexafluoride isinjected into the process chamber and a butterfly valve above the turbopump is throttled to achieve the operating pressure of 2 to 5 mT.Subsequently, power is applied to generate a plasma. This system usestwo radio frequency (RF) power sources. One is connected to the chuckand is used to control energies of ions reaching the substrate and isset between 1 to 2 W/cm². The second RF source is connected to a threeturn inductor coil wrapped around the ceramic process chamber. Thesecond RF source provides the main plasma generating power, controlsplasma densities and is set between 800 to 1200 W.

Prior to etching the via, the SiC substrate is coated with ITO, thenpatterned with photoresist using standard photolithography. The ITO isthen dry etched in chlorine chemistry in which the photoresist is theetch mask. Vias are subsequently etched in SiC in fluorine chemistry inwhich the ITO is the etch mask. The via dry etch process is highlyanisotropic, with SiC etch rate of 0.5 to 0.8 micron/min, andselectivity to the etch mask of 100 to 150.

In the specification, there have been disclosed typical embodiments ofthe invention, and, although specific terms have been employed, theyhave been used in a generic and descriptive sense only and not forpurposes of limitation, the scope of the invention being set forth inthe following claims.

That which is claimed is:
 1. A method of etching a via in siliconcarbide comprising: masking a silicon carbide target substrate with amasking layer selected from the group consisting of indium-tin-oxide(ITO) and magnesium oxide; etching the masked silicon carbide substratein an inductively coupled plasma formed substantially entirely from afluorine-containing gas; and applying a chuck power to the substratethat maximizes the silicon carbide etch rate and maximizes thedifference between the etch rate of the silicon carbide and the etchrate of the mask in the inductively coupled plasma.
 2. A methodaccording to claim 1 comprising exposing the wafer to a plasma formedfrom sulfur hexafluoride gas.
 3. A method according to claim 1comprising etching the substrate in a plasma formed from sulfurhexafluoride gas; masking the target substrate with ITO; and applying RFpower to the target substrate in an amount sufficient to realize etchrates of about 0.5 microns per minute in the silicon carbide, but lessthan an amount at which the ITO mask begins to significantly erode.
 4. Amethod according to claim 3 wherein the RF power is applied to a chuckholding the target substrate in an amount of about 1 to 2 watts persquare centimeter of the chuck.
 5. An etching method according to claim4 wherein the inductively coupled plasma is formed by directing betweenabout 800 and 1200 W of power to an inductive coil of an ICP applicator.